In recent years, musical sound reproducing apparatuses having a function of outputting audio signals as digital signals have been widely adopted. A digital signal from such a musical sound reproducing apparatus is converted to sound waves, for example, by an electroacoustic transducer apparatus (hereinafter referred to as “digital electroacoustic transducer apparatus”) that can output desired sound waves according to a digital signal (see Japanese Patent Laid-Open No. 2015-065661, for example).
Examples of such digital electroacoustic transducers apparatus include speakers installed indoors, and earphones and headphones worn over a user's ears (head).
The digital electroacoustic transducer apparatus includes a dynamic drive unit and a signal processing circuit for generating processing signals based on digital signals from a sound source. The drive unit includes a diaphragm and a plurality of voice coils. Each voice coil is driven with a processing signal generated in the signal processing circuit. Consequently, the digital electro-acoustic transducer efficiently generates sound at high to low frequencies based on audio signals.
A known electroacoustic transducer apparatus that converts audio signals to sound waves includes a system for canceling noise in the external environment (hereinafter referred to as an “NC system”).
An electroacoustic transducer apparatus including the NC system includes a microphone and a noise canceling circuit (hereinafter referred to as an “NC circuit”). The microphone picks up noise around the electroacoustic transducer apparatus and generates a noise signal. The NC circuit generates a cancel signal according to the noise signal generated by the microphone. The cancel signal is a signal acoustically opposite in phase to the noise signal. The electroacoustic transducer apparatus generates sound waves based on a synthesized signal generated by synthesizing the cancel signal and an audio signal. Consequently, noise is acoustically canceled out with sound waves generated based on the synthesized signal through the electroacoustic transducer apparatus, and is thus canceled.
When such an NC system is mounted in a digital electroacoustic transducer apparatus, signal processing, such as generation of a cancel signal and synthesis of a cancel signal and an audio signal, is executed, for example, through digital processing using a digital signal processor (DSP) (see Japanese Patent Laid-Open No. 2017-098993).
The NC system disclosed in Japanese Patent Laid-Open No. 2017-098993 executes generation of a cancel signal and generation of a synthesized signal through a single DSP. Therefore, the NC system can generate an appropriate cancel signal depending on the type of noise.
Generating a cancel signal through digital processing using a DSP increases the time required for generating the cancel signal according to the amount of computation in the DSP to delay in time. Consequently, the phase of the cancel signal is not opposite to the phase of noise to be canceled, and is delayed by a phase corresponding to the time delayed with respect to the phase opposite to that of the noise, causing a phase difference between the cancel signal and the phase opposite to that of the noise.
In addition, when a synthesized signal is generated by digital processing using an adder circuit (mixer) included in the DSP, the phase of the synthesized signal (cancel signal) varies depending on the phase characteristics of the adder circuit. Consequently, the phase of the cancel signal is not opposite to the phase of the noise to be canceled but is shifted by the phase change from the opposite phase, causing a phase difference between the cancel signal and the phase opposite to that of noise.
Thus, if a phase difference occurs between the cancel signal and the phase opposite to that of the noise, the cancel signal cannot cancel the noise sufficiently by canceling it out.